Found an unlocked AT&T Bold on eBay, I have a question
 

I'm already an AT&T customer and I found a Bold on eBay that the seller listed as "unlocked." It's an AT&T Bold. If I were to upgrade the OS down the road.. It shouldn't matter right? Can someone explain to me what exactly is done to unlock and how it affects the phone besides being able to use it on other networks. Would upgrading the OS "lock" the phone to the original carrier (AT&T)?

updating the OS could lock the phone but of coarse it would not matter since you are already an att customer

No. the unlock is based on the imei no. and the OS has no affect on this.
the unlock means that you can use it on any network, and that is all.

Wrong.
Once unlocked, always unlocked.

An unlock will release your phone from the carrier that issued it. It will allow you to use other carriers by switching the SIM card. Since it is a GSM device you can only use it on GSM carriers. Once it's unlocked upgrading or downgrading the OS will have no effect on it. As Penquin stated "Unlocked IS unlocked".

Be careful and read the before you buy sticky at the top of the Buy, Sell, Trade forum here at BBF. Good tips for making a smooth purchase!

E-

Before you get it, read this. I hope it helps your Ebay purchase. Buying a BlackBerry off eBay - BlackBerryFAQ

Should be good. The seller claims he just purchased the phone as an upgrade and put it up on eBay to make a few extra bucks.

I would still check it out first!

Unlocking merely allows the carrier branded devices to accept non-carrier branded sims. And thereby allowing you to connect to different carriers/gsm networks as a result.